This invention relates to high voltage transformers for video display apparatus and, in particular, to high voltage transformers that utilize a tertiary winding to generate a focus voltage.
The electron gun assembly of a color cathode ray tube produces one or more electron beams which impinge upon a phosphor display screen in a predetermined pattern to form a scanned raster. The electron gun assembly is designed to produce a number of spatial regions of different voltage potentials through which the electron beam or beams pass. One of these voltage potential regions provide focusing of the beams so that the spots formed when the beams strike the display screen are of a desirable size and sharpness.
The focus voltage or potential may be generated by providing a tap on the high voltage or tertiary winding of the high voltage transformer. The electron gun assembly used in one prior art utilizes a high voltage winding tapped to provide a focus voltage nominally equal to one-third of the high voltage or ultor potential. Changes in electron beam current, due to variations in picture brightness, may require that the focus ratio (i.e., the ratio of the focus voltage level to the high voltage level) remain constant regardless of the loading on the high voltage winding. As the electron beam current increases, however, the loading on the high voltage supply also increases, which may cause the high voltage level to decrease, resulting in an increase in the focus ratio. Some picture tubes incorporate an electron gun assembly that, in order to produce optimally focused beams, require the focus ratio to remain constant as beam current increases.